Targets are usually disk or rectangular shaped plates, which serve as sources of substances to be sputtered on substrates such as wafers to form thereon electrodes, gates, conductor lines, elements, insulating films, protective films and the like of various semiconductor devices. As accelerated ions impinge on a target surface, the atoms constituting the target are partly released by momentum exchange from the target surface to the ambient space to deposit on an oppositely located substrate. Typical sputtering targets in use include the targets made of Al and Al alloys, refractory metals and their alloys (W, Mo, Ti, Ta, Zr, Nb, etc. and their alloys such as W-Ti), and metal silicides (MoSi.sub.x, WSi.sub.x, NiSi.sub.x etc.).
Generally, targets of two or more different elements are made by alloying or compounding those elements by melting or powder metallurgy. Where a target of good quality is not obtained by these methods because of the density, composition, oxygen concentration, or other problem, the adoption of a mosaic target system is often an alternative. The mosaic target system is realized, when a target is to be formed of two or more elements, by working the materials of individual elements or their alloys or compounds to suitable shapes and appropriately combining and arranging them to form altogether a disk or rectangular plate as a target of a desired composition. The target thus combined and arranged is called a mosaic target. In the mosaic target, the appropriately shaped pieces of each element or its alloy or compound which constitute the structural units are arranged in a combination in abutting relation. These structural units are called as target block pieces. For example, in a Ta-Mo mosaic target, Ta block pieces and Mo block pieces are alternately combined and arranged in a stripe pattern or in a radial pattern.
Examples of mosaic targets, besides the Ta-Mo target, are targets of alloys of refractory metals such as Ta-W, Ta-Ti and W-Ti, and silicide targets.
Conventional mosaic targets are fabricated by putting the target block pieces together in abutting relation and bonding them in that abutting state to a backing plate with the use of a solder, namely a brazing metal such as In or the like or by shrink fitting them directly on and into a backing plate. In either case, gaps of several to several hundred microns are formed among the adjacent target block pieces at their abutting interfaces. The gaps can cause abnormal discharge, localized acceleration of sputtering rate that induces particle generation, or concurrent sputtering of the solder or backing plate, leading to contamination and deterioration of the characteristics of the resulting sputtered film on a substrate.
The term "particles" as used herein means the particulate matters that scatter within a film forming apparatus during thin film formation by sputtering and deposit in clusters on a substrate in the apparatus. The clustered particles often grow to sizes as large as several microns across. Their deposition on a substrate can cause problems, e.g., shorting or breaking of conductor lines in large-scale integrated circuits, resulting in an increased percentage of rejects. These particles originate from the particulate matters released from the target.
To avoid concurrent sputtering of the solder or backing plate, methods of working the block pieces have been adopted so that their abutting interface profiles are stepped or slanted as at (a) and (b), respectively, in FIG. 3. Here the target block pieces of materials A and B are alternately combined with their stepped or slanted interfaces in an abutting relation and bonded to a backing plate so that the backing plate and the solder are concealed from the surface to be sputtered. However, these approaches still are unable to control abnormal discharge and particle generation. Moreover, the additional working of the abutting interfaces of target block pieces reduce the yield, and in the targets that use expensive materials the material cost increases, and further, working cost is expensive too. In some target materials, even such additional working is difficult to perform.
It is demanded to develop a mosaic target which precludes the possibility of abnormal discharge or particle generation due to gaps at the abutting interfaces of mosaic target block pieces without requiring unwanted and superfluous working of the target material.
Needless to say, means to be taken must be one which can prevent the deterioration of quality characteristics of individual target block pieces such as crystal grain growth, the change of crystal orientation and others and cause little deformation or other adverse effects upon the target block pieces.